Submitted to: American Society of Civil Engineers Water Resources Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/1/1999
Publication Date: N/A
Interpretive Summary: The repercussions from bank failures include flooding, land and fish habitat loss. Understanding failure processes will allow the root causes of erosion to be tackled. The paper discusses a concept that suggests that failed blocks of material will collect at the bottom of the bank and protect the bank face from future erosion. This may reduce the detrimental effects caused by failures. An equation was used to estimate the amount of resistance that fallen block materials offer against removal by flows in a Mississippi creek. It takes into account sediment weight. However, besides the block weight, a block puller device measured another form of resistance, called cohesion, which also prevents failed materials from being washed away. Part of the cohesion is due to suction and this is lost when the area considered is saturated. It is therefore possible to estimate the resistance to removal of such blocks when they are submerged by a high flow. If the opposing drag force of the flow is less than this resistance the blocks will not move and they will protect the bank face which they buttress. It was found that some blocks would theoretically remain in place, despite peak flows, only if the resistance due to cohesion was considered.
Technical Abstract: Failed bank materials may act as a natural form of bank-toe protection. Fluvial erosion of bank-toe materials is a pre-requisite for continued bank undercutting and retreat. The input volume of sediment to a bank's basal region and the dimensions of individual failure blocks can be monitored. However, further investigations in the bank-toe zone are necessary to allow a complete analysis of the controlling processes that contribute to disintegration or removal of such basal support. To date, block entrainment is predicted from a Shields diagram as a function of block size and bed roughness. Investigations in Goodwin Creek, Mississippi, revealed apparent cohesion between blocks and their underlying surface. The method used to assess this cohesion involved a pulley system. Blocks are attached to a load cell using a strap before the pulley is wound up. The load cell measures the force against gravity including the block weight and any apparent cohesion. Apparent cohesion of 2.65 kPa was measured, identifying a pressure source that bonds blocks to the underlying surface. Cohesion values and types vary spatially and temporally. Tensiometric tests beneath blocks suggest that cohesion due to matric suction alone may be as much as 1.8 kPa in winter. Apparent cohesion is believed to help prevent removal of the largest blocks by a peak flow of 66.4 m**3/s on 9/23/97. It seems insufficient to estimate block entrainment in the basal area from block size or bed roughness alone, as in a Shields-type approach.